In the prior art it is known, for example from the documents DE 196 52 950 A1 or EP 0 849 112 A1, to charge a traction battery by means of the on-board drive system of a motor vehicle at an external AC or three-phase current source. In this case the inverter of the drive system is used as a step-up converter in the direction toward the battery. When an asynchronous machine is used as the drive aggregate, then—to avoid a high reactive power demand—the machine is idled during the charging process and can advantageously be used to lower the power supply voltage, for example by means of suitable winding taps, i.e. so as not to endanger the battery. The short-circuited rotor winding or short-circuited rotor of the asynchronous machine is opened by means of switches for charging the battery.
The disadvantage of the arrangement adopted in the prior art is that switches have to be provided which, when the traction battery is to be charged, interrupt the otherwise short-circuited rotor winding. Such power switches incur additional costs and add to the weight.